Rabbit cytochrome P-450, isozyme 5, is highly active in the metabolism of some aromatic amines to mutagenic products. This isozyme, which we initially purified from rabbit lung, can be detected by immunochemical methods in several other tissues including liver and bladder. Following treatment of rabbits with phenobarbital, the hepatic concentration of protein detected by antibodies to isozyme 5 increases about 10-fold; however, no increase in this protein is noted in the lung. Thus, the response of isozyme 5 to phenobarbital is the same as that of isozyme 2 although these two forms of cytochrome P-450 do not appear to be immunochemically or functionally related. We have now purified isozyme 5 from the livers of rabbits treated with phenobarbital and compared it with isozyme 5 from lung. The proteins from liver and lung have been digested with several proteolytic enzymes. No differences are noted when the peptides formed are compared by SDS-gel electrophoresis and "Western blotting". The proteins from liver and lung are also the same with respect to UV-vis spectra, the sequence of the first 20 N-terminal amino acids, and their catalytic activities with 2-aminofluorene. In contrast to isozyme 5 of cytochrome P-450, the flavin-containing monooxygenases of rabbit liver and lung are different enzymes. The relative contributions of cytochrome P-450, isozymes 4 and 5, and the flavin-containing monooxygenase to the metabolism of 2-aminofluorene in rabbit liver, lung and bladder are now being investigated.